Traditionally soap bars are stamped from extruded billets of material in order to achieve bars of an attractive and uniform appearance. A common problem in bar stamping is however that of die blocking. Die blocking occurs when small particles of soap transfer from the billet surface and adhere to the die surface.
The initial small particle of soap can act as a nucleus for the adhesion of more soap material from subsequently stamped billets until the adhering collection of soap material is sufficiently large to impart visually perceptible indentations in further stamped soap bars.
Conventionally a pair of die members employed in stamping of soap bars follows a stamping cycle controlled by a simple cam mechanism. In order to aid the mechanical workings and longevity of the stamping machine the cycle will be fixed. The velocities of approach and release from the bar will normally follow a simple sinusoidal cycle.
In such a case the relative velocity V of the dies after initial contact with the billet is approximately related to the distance h between the dies by an equation of the form EQU V=kh.sup.0.5
where k is a constant.
The equation is approximate, because the velocity V must of course reduce to zero as the dies reach their position of closest approach. At this position the dies do not normally touch. There remains a small gap between them through which surplus soap is extruded as a flashing.
Because of this relationship between V and h, V decreases less rapidly than h. For instance as h drops from an arbitary value h.sub.a to 1/2h.sub.a, the magnitude of V drops from EQU V.sub.a to K(1/2h.sub.a).sup.0.5 =(1/2).sup.0.5 V.sub.a =0.71 V.sub.a
This can be expressed as a statement that V decreases less than linearly with distance.
Typically the relative velocity V of the dies at the point of the cycle where they come into contact with the soap billet is 300 mm/sec.
W088/7572 and W088/7573 discloses soap stamping machinery in which dies are moved by a programmable actuator. These documents envisage that the dies will be caused to move according to predetermined rules concerning position and speed of movement. However, the nature of such rules is not stated.
The conventional sinusoidal cycle may be optimum from the point of view of operation and wear and tear of the stamping machine but we have found that it is not optimum having regard to reducing die blocking on the resulting soap bars. Die blocking occurs due to the adhesive forces existing between the soap bar surface and the die member surfaces as the dies separate after stamping.
We have found that a significant factor affecting the magnitude of these adhesive forces and hence the extent of die blocking is the magnitude of the maximum compressive force between the closing dies and the soap. We have found that a reduction in die blocking can be achieved by imposing a different relationship between the relative velocity of the dies and the distance between them, so as to reduce the maximum compressive force.
If stamping is carried out with apparatus where the velocity is related to distance by a formula such as EQU V=kh.sup.0.5
as in conventional cam driven machinery, we have found that the compressive force between the soap and the dies rises progressively as the dies close together, and becomes very high at the end of their stroke. This of course signifies that the maximum compressive force could be reduced simply by slowing the machinery. However, that would reduce overall production.
However, if the relationship between velocity and distance is changed so that the dies' relative velocity is made to drop more sharply during at least part of their travel after contacting the billet, their velocity during the latter part of their travel becomes a smaller fraction of their velocity when they contact the billet. As a result the maximum compressive force is reduced without bringing about as great a reduction in the overall rate of stamping. According to a first aspect, the present invention provides a process for stamping soap and/or detergent bars in which a billet is located between a pair of opposing die members which are urged together to stamp the billet into a bar, characterised in that for at least part of the travel of the dies from initial contact with the billet to their point of closest approach, their relative velocity (V) decreases at a rate which is more than proportional to the change in the distance (h) between the dies.
With this relationship between velocity and distance, the velocity V.sub.a at a distance h.sub.a and the velocity V.sub.b at a later distance h.sub.b will be related by the inequality ##EQU1##
Preferably the relationship between velocity and distance may approximate to EQU V=k'h.sup.n
where k' is a constant and n is at least 1.5 better 2 or more. In particular it is preferred to approximate such a formula in which n=3. Then the compressive force will remain roughly constant as distance h decreases.
Such a relationship has the consequence that velocity and distance satisfy an inequality: ##EQU2## where m.gtoreq.1.5, e.g. m.gtoreq.2.
By arranging relative velocity to follow such a relationship to distance it is possible to achieve a reduction in the maximum compressive force while increasing the relative velocity of the dies at the moment of initial contact with the billet, and so not increasing the overall stamping cycle time.
The invention thus leads to a reduction in the occurrence of die blocks, yet the overall rate of production does not need to be diminished unacceptably, and indeed may not be diminished at all.